One of electromechanical transducers is a capacitive micromachined ultrasound transducer (CMUT). In general, CMUTs include a substrate having a lower electrode, a vibration membrane supported by a supporting unit formed on the substrate, and an upper electrode formed on the vibration membrane. The lower electrode faces the upper electrode with a gap therebetween. A structure including the vibration membrane, the upper electrode, and the lower electrode for a gap is referred to as a “cell”, and one or more cells electrically connected to one another are referred to as an “element”. In a CMUT, a vibration membrane is vibrated by received ultrasound waves, and the ultrasound waves are detected by using a variation in capacitance.
A CMUT includes an element array in which a plurality of elements are arranged in an array. Each of the elements transduces received elastic waves into an electrical signal. However, the characteristics of the elements differ from each other. The differences cause a variation in the sensitivity of CMUTs. In order to detect a variation in sensitivity, PTL 1, for example, describes a method for transmitting ultrasound waves having a single frequency from an ultrasound source. In PTL 1, each of the elements receives the ultrasound waves. By using electrical signals transduced by the plurality of elements, the sensitivities of the elements are detected.